This invention relates to a droplet generator for a continuous stream ink jet print head.
More particularly the invention relates to such a generator comprising: an elongate cavity for containing the ink; nozzle orifices in a wall of the cavity for passing ink from the cavity to form jets, the nozzle orifices extending along the length of the cavity; and an actuator disposed on the opposite side of the cavity to the nozzle orifice wall for vibrating the ink in the cavity such that each jet breaks up into ink droplets at substantially the same predetermined distance from the wall, i.e. such that there is uniform jet break up.
Droplet generators of the above kind are disclosed in U.S. Pat. No. 4,587,528 and U.S. Pat. No. 5,502,473. However, in order to achieve uniform jet break up, these generators must be operated at a frequency at which the ink cavity is resonant in its vertical dimension, i.e. from the nozzle orifices to the actuator. In other words, it is necessary that at operating frequency an integer number of half wavelengths fit precisely into this dimension. Consequently, a very high accuracy is required of the physical dimensions of the structural components of the generator. Further, very little stray is permitted in operating parameters of the generator such as ink composition and temperature.
WO-A-98/51503 also discloses a droplet generator of the above kind, which generator is able to achieve the required uniform jet break up without resonance of the ink cavity in its vertical dimension. This is done by arranging for the actuator to address the full or complete area of the face of the ink cavity opposite the nozzle orifice wall. The actuator addresses the ink in a piston-like manner, i.e. all points across the actuator face that addresses the ink vibrate vertically in phase and with the same amplitude. A drawback with this droplet generator is that it is difficult to achieve the required precise piston-like motion of the actuator at all points along the actuator's length. This problem increases the longer the ink cavity (typically 50 mm and above) and the higher the frequency of operation (typically 100 kHz and above).